Techniques for preventing secret information printed on a print medium from being leaked have been developed in recent years. Among others, techniques have been proposed in which an image that one does not want any unauthorized persons to see is encrypted in advance and the thus encrypted image is printed on a medium such as paper. An encryption device that uses one such technique first encrypts a portion of an input image and orderly converts the values of the pixels contained in the encrypted region, and then generates a unique pattern corresponding to the pixel value conversion. Further, the encryption device attaches a location marker for locating the encrypted region to at least two of the four corners of the encrypted region. The encryption device further attaches a checking marker which is used to verify the validity of the decrypted image obtained by decrypting the encrypted region. On the other hand, a decryption device, using a reading device such as a scanner, reads a medium on which the image having the encrypted region is printed, and recovers the original image by decrypting the encrypted region by referring to the location markers on the thus captured image.
Patent document 1: International Publication Pamphlet No. WO 2008/053576